This invention generally relates to optoelectronic devices and more particularly relates to vertical cavity lasers (VCLs) having high reflectivity, and low loss dielectric or hybrid distributed Bragg reflectors.
A VCL is a semiconductor laser consisting of a semiconductor layer of optically active material, sandwiched between highly-reflective layers of metallic material, dielectric material, epitaxially-grown semiconductor material or combinations.
Conventional VCL designs utilize a thin active region, typically on the order of one wavelength of the emitted light, to achieve a low threshold current as well as longitudinal (or axial) mode control. However, thin active regions typically have a single pass optical gain of approximately 1%, so that upper and lower mirrors having reflectivities greater than about 99% are required to achieve lasing.
Conventional VCL designs often utilize semiconductor distributed Bragg reflectors (DBRs) to achieve the required reflectivities. DBRs as is known in the art comprise a plurality of pairs of quarter-wavelength thick layers, each pair comprising a relatively high refractive index layer adjacent a relatively lower refractive index layer. Depending on the refractive index difference between these layers, just a few pairs of layers can yield reflectivities greater than 90% over a wavelength range of several hundred nanometers.
In practice dielectric DBRs or hybrid DBRs comprising a semiconductor mirror portion and a dielectric mirror portion may provide significant advantages over semiconductor DBRs in terms of performance and manufacturability. For example, dielectric or hybrid DBRs may provide the same reflectivity with lower loss, higher efficiency and increased longitudinal optical confinement. Conventionally the index difference at the interface between the semiconductor and dielectric materials provides a high reflection but may also result in significant loss due to scattering at the interface or the presence of a highly doped or absorbing contact layer.
In an exemplary embodiment of the present invention a vertical cavity laser includes a hybrid optical cavity adjacent to a first mirror, the optical cavity comprising a semiconductor portion and a dielectric spacer layer. A dielectric DBR is disposed adjacent to the dielectric spacer layer. The interface between the semiconductor portion of the optical cavity and the dielectric spacer layer is at or near a null in the associated optical standing wave intensity pattern to reduce the losses or scattering associated with that interface.
In another aspect of the present invention a vertical cavity laser includes an optical cavity adjacent to a first mirror, a hybrid mirror having a semiconductor mirror adjacent to the optical cavity, a dielectric spacer layer adjacent to the semiconductor mirror and a dielectric DBR adjacent to the dielectric spacer layer, wherein the interface between the semiconductor portion of the hybrid mirror and the dielectric spacer layer is at or near a null in the optical standing wave intensity pattern.
In another aspect of the present invention a method for forming a vertical cavity laser includes the steps of forming a hybrid optical cavity adjacent to a first mirror, forming a dielectric spacer layer adjacent to the semiconductor portion of the optical cavity and forming a dielectric DBR adjacent to the dielectric spacer layer, wherein the interface between the semiconductor portion of the optical cavity and the dielectric spacer layer is at or near a null in the optical standing wave intensity pattern of the vertical cavity laser.
In a further aspect of the present invention an optical subassembly includes an electrical package containing a VCSEL. The VCSEL has a hybrid optical cavity adjacent to a first mirror, wherein a dielectric spacer layer is adjacent to the semiconductor portion of the optical cavity, and a dielectric DBR adjacent to the dielectric spacer layer, wherein an interface between the semiconductor portion of the optical cavity and the dielectric spacer layer is at or near a null in optical standing wave intensity pattern of the vertical cavity laser. The package may further include a housing attached to the electrical package, the housing including a ferule for aligning a fiber with an optical path carrying light from the VCSEL.